1. Field of the Invention
The present invention relates to the field of plastic substrates for magneto-optic records and more particularly to the field of magneto-optic records having substrates molded from plastic so as to have low variation in birefringence.
2. Description of the Prior Art
Magneto-optic (MO) recording technology is under intensive development by numerous companies in the United States, Europe, and Japan, because of the expectation of combining, in a removable MO record, the advantages of high track density associated with optical recording with the erasability associated with conventional magnetic disk recording.
As understood, the normal practice in the art of manufacturing a substrate for an MO record is to injection mold transparent plastic material that is selected as having high optical quality. Difficulties have been experienced in finding plastic materials having both high optical quality and the mechanical properties that are desired when the substrate both supports an MO thin film stack and transmits optical energy to such stack for recording and reading. For example, plastics such as polymethylmethacrylate (PMMA) can be molded so as to have excellent optical quality, but are undesirable with respect to such mechanical properties as dimensional stability in the presence of temperature, humidity extremes and centrifugal stresses. As an example, PMMA is quite permeable to moisture. If moisture permeates the substrate it can weaken the adhesion between the substrate and the active thin film structure, which tends to decrease the potential useful life of the record. Moisture permeation also causes the substrate to expand, further aggravating the adhesion and film cracking problem. Thus, those developing records based on this new MO recording technology have attempted to use plastics other than PMMA, such as polycarbonate (PC), which have better mechanical properties, such as dimensional stability and lower permeability to moisture. However, as compared to PMMA, PC substrates for MO records have less desirable optical properties in that the birefringence of PC substrates is greater than that of PMMA substrates. Birefringence in plastic MO media refers to the dependence of the index of refraction of the molded plastic substrate material upon the direction of the plane of polarization of light transmitted through the substrate. This birefringence is related to stresses in the plastic, usually created during the injection molding process and "frozen in".
Birefringence is undesirable in MO recording. In particular, as usually practiced in magneto-optic (MO) recording, a focussed beam of light or near-infrared radiation is focussed through the transparent plastic substrate of the MO record onto an active layer of the MO thin film structure. The active layer is magnetic material, the magnetization of which is all in one direction perpendicular to the plane of the substrate when the MO record is originally prepared prior to making a first recording. The beam of light or radiation, referred to as a recording beam, is effective, in the presence of a small magnetic bias field in the direction opposite to that of the magnetization, to heat minute recording locations of the active layer. This results in reversing the magnetization direction at such heated locations, since the magnetization there becomes aligned with the direction of the bias field. In such practice, the substrate provides not only support for the thin film structure, but dust defocussing of small particles of dirt to improve the accuracy of reading the minute recording locations.
The MO record is read by another light beam, the read beam, which may be the same as the recording beam but having reduced intensity and not being modulated. The read beam, which is plane polarized, is focussed on and is reflected by the active layer. Depending on the direction or orientation (up or down) of the magnetization at a given location, the plane of polarization of the read beam is rotated slightly clockwise or counterclockwise. This polarization rotation is referred to as the "polar Kerr magneto-optic effect". Birefringence in the substrate causes an unwanted, spurious additional rotation of the plane of polarization of the read beam, which adds to the signal polarization rotations produced by the magneto-optic Kerr effect, thus adding an undesired distortion, or "polarization noise" to the detected MO signal.
In attempts to process detected MO signals from MO records that include substrates having birefringence that varies from one recording location to another on the record, it has been found that the effect of such varying birefringence is to add to the signal both high frequency noise-like distortions and low frequency pedestal variations. Signal processing, such as by electronic high pass filtering, can separate the signal from low frequency components, but cannot separate the noisy components within the signal bandwidth.